1. Field of the Invention
The present invention relates to a control method for a positive temperature coefficient (PTC) heater and to an air conditioner including the PTC heater.
2. Description of Related Art
A conventional air conditioner is disclosed in Japanese Patent Application Laid-open No. Hei 08-152179. The air conditioner has an integrated structure in which an indoor unit to be placed indoors is disposed in the front and an outdoor unit to be placed outdoors is disposed in the rear. In the outdoor unit, there are disposed a compressor for operating the refrigeration cycle and an outdoor heat exchanger connected to the compressor. The indoor unit has an inlet and an outlet opened therein and, inside the indoor unit, there are disposed an indoor heat exchanger connected to the compressor via a refrigerant pipe, and a heating portion including a PTC heater.
When starting cooling operation, the refrigeration cycle is operated by the drive of the compressor, and the indoor heat exchanger serves as an evaporator on the low temperature side in the refrigeration cycle while the outdoor heat exchanger serves as a condenser on the high temperature side in the refrigeration cycle. The air in a room flows into the indoor unit from the inlet to be subjected to heat exchange with the indoor heat exchanger so that the air thus cooled is delivered to the room from the outlet. This way, cooling in the room is performed.
When starting heating operation, the refrigeration cycle is operated by the drive of the compressor, and the indoor heat exchanger serves as a condenser on the high temperature side in the refrigeration cycle while the outdoor heat exchanger serves as an evaporator on the low temperature side in the refrigeration cycle. The air in a room flows into the indoor unit from the inlet to be subjected to heat exchange with the indoor heat exchanger and is thereby heated. The air flowing into the indoor unit is further heated by the drive of the heating portion. The air thus heated is delivered to the room from the outlet, to thereby perform heating in the room.
The PTC heater of the heating portion is formed such that a heating element having PTC characteristics is sandwiched by electrodes, and is driven through application of a voltage between the electrodes. When the temperature of the heating element exceeds the Curie point, the heating element shows a sudden increase in resistance to reduce a current value and a heating amount thereof. Accordingly, a stable amount of heating in the heating portion is obtained to make it easy to generate warm air of a predetermined temperature and also prevent overheating.
In this case, the PTC heater is low in temperature when starting up and accordingly the heating element is low in resistance, which leads to a risk that an overcurrent flows to exceed power capacity. It is known to contain a component having negative temperature coefficient (NTC) characteristics in the heating element in order to suppress the overcurrent at the time of start-up. However, the component having the PTC characteristics and the component having the NTC characteristics have different coefficients of thermal expansion, which accelerates the characteristic deterioration in the PTC heater.
As a countermeasure, Japanese Patent Application Laid-open No. 2003-59623 discloses a control method in which a current flowing through the PTC heater at the time of start-up is monitored to control the drive of the PTC heater so that the power capacity is not exceeded. Specifically, the PTC heater is subjected to triac control, in which duty control of varying a pulse width of a gate signal to a triac is carried out.
The PTC heater starts to be driven with the pulse width of the gate signal set to 0, and thereafter, the pulse width is increased by 1 bit at a time. Then, when a current value of the PTC heater reaches a predetermined allowable range, increasing the pulse width is stopped, and when the current value exceeds the allowable range, the pulse width of the gate signal is decreased. On the other hand, when the current value falls below the allowable range, the pulse width is increased. This way, the current flowing through the PTC heater makes a transition within the allowable range, to thereby prevent the overcurrent at the time of start-up.
However, in the above-mentioned drive control on the PTC heater disclosed in Japanese Patent Application Laid-open No. 2003-59623, the PTC heater has a significantly low initial temperature in some cases depending on ambient temperature or an air flow rate. In such a case, if the pulse width of the gate signal to the triac is increased at an advanced timing, an overcurrent flows through the PTC heater, causing a problem that the power capacity is exceeded and the circuit breaker trips.